Beginners Guide to Astronomy
Amateur Astronomy is one of the most fascinating hobbies in the world, but to get really into it, you probably will want to get a telescope either for yourself or perhaps for a son or daughter who is showing an interest in the wonders of the universe. Somehow, you will have to find your way through the maze of ads, catalogues, claims and counterclaims about which telescope is best or the least expensive. You may be asking yourself questions like “What type should it be?”, or “Just what should I believe when I read the ads?” Finally, you will have to lay out the cold hard cash needed to purchase what you hope will be the best instrument your money can buy. If you are a bit confused or frustrated at this point, don’t worry. I know exactly how you feel, because I have gone through this many times before. This article should help answer many of the questions you may have about purchasing a telescope as well as clearing up some of the mysteries about astronomical telescopes in general.
A telescope has two main functions, to gather and focus a large amount of light from an object (much more than the eye alone can) to form an image of the object, and to magnify that image so that distant objects can be better seen. There are many different designs of telescope that will accomplish these functions but only three designs are practical for small inexpensive telescopes: the Refractor, the Newtonian Reflector and the Catadioptric. A refractor focuses light by “refracting” or bending it through a special two element glass lens, a reflector focuses light by reflecting it off a curved mirror and a catadioptric uses a combination of both lenses and mirrors. A set of lenses known as the Eyepiece then magnifies the focused image in each design.
Types of telescope
Refractor
The refractor is the instrument that conforms to the general public’s mental image of a telescope possessing a long brass tube with a big lens at the front with an astronomer in a white coat peering into an eyepiece at the other end. In reality, the lens at the front of the tube is not one piece of glass, but two (or more) glass elements sandwiched together in order to produce an image virtually devoid of false prismatic colour.
The tubes of so-called achromatic refractors are generally 12 to 15 times longer than the diameter of the front lens, though modern apochromatic (multi-lens element) designs are somewhat more manageable with tubes 6 to 9 times the diameter of the objective lens. What is really being discussed when speaking of tube length is focal length — the distance from the lens at which an image is formed of a distant object. The ratio of focal length to aperture is known as the focal ratio. For example, a 10cm lens with a focal length of 120cm has a focal ratio of 12 (written f/12).
Reflector
The reflecting telescope, on the other hand, uses specially curved mirrors to gather and focus the light from the object under scrutiny. The most commonly encountered telescope of this type is the Newtonian, designed by Sir Isaac Newton in 1668. Light from the object enters the tube to fall on the concave primary mirror at the base. The light then converges to a focus near the top of the tube where a small flat mirror diverts the light out to the side where the image is inspected with an eyepiece. Unlike the refractor, the reflector is perfectly achromatic in performance and its focal length is rarely more than 8 times the diameter of the primary mirror (f/8). This means that even large aperture Newtonians can have tubes of manageable proportions.
Catadioptric
Both the refractor and reflector have pros and cons, but out of a desire to marry the best aspects of both the catadioptric telescope was born early this century. In this form of telescope both the lens and the mirror are used to produce instruments of large aperture that are virtually devoid of false prismatic colour in a short tube. The most commonly encountered telescope in this category is the so-called Schmidt-Cassegrain (or Maksutov-Cassegrain) that uses a specially shaped lens at the front to correct for the errors of the short focus primary mirror. The latter’s image is magnified by and reflected back to a final focus through a hole in the primary by a convex secondary mirror. This produces a telescope with an equivalent focal length 10 to 15 times the primary mirror’s diameter squeezed into a tube less than a fifth of that length.
Telescope mountings
Once an object, whether terrestrial or astronomical, is located and centered in the telescope’s field of view, the telescope’s mechanical mounting permits the observer to track, or follow, the object as it moves across the landscape or sky. Types of telescope mountings include the following:
Altazimuth Mountings
The simplest type of telescope mount allows the telescope to be moved up-and-down (in vertical, or altitude) and left-to-right (in horizontal, or azimuth). The altitude-azimuth (altazimuth) mounting thus permits the observer to follow objects by simple motions of the telescope in vertical and horizontal. Slow-motion controls, sometimes operated through flexible cables, can facilitate these motions. The altazimuth mount, owing to its simplicity and relatively lower cost, is widely used with telescopes in both land-viewing and astronomical applications.
Equatorial Mountings
Although celestial objects are essentially fixed in their positions in the sky (on the celestial sphere, the imaginary spherical surface on which all astronomical objects are located), they appear to move in an arc across the sky, as the earth rotates underneath the sky once every 24 hours. From an astronomical point of view, therefore, the task of the telescope mounting is to compensate for the Earth’s rotation and allow the observer to track the Moon, planets, and stars. This task is made vastly easier by the equatorial mounting, the type of mounting incorporated into larger or more advanced telescopes. By aligning one axis of the equatorial mount to the Earth’s rotational axis (a simple process which involves pointing one telescope axis to the North Star), the observer can track astronomical objects by turning one control cable, instead of the two simultaneous motions required with the altazimuth mount. If a small motor is attached to the equatorial mount, this tracking can be performed automatically. These motor drives are available for most equatorially mounted telescopes.
The German equatorial mount is most commonly encountered form for refractors and Newtonian reflectors. Very often the declination and polar axes will not only be equipped with slow motion controls, but graduated setting circles as well. These graduated circles act like protractors so that you can effectively ‘dial-in’ the co-ordinates of celestial bodies in a manner analogous to looking up the latitude and longitude of a place on the Earth’s surface. The more upmarket equatorials will also be equipped with an electric motor of some description that will drive the telescope about the polar axis keeping your desired object in the field of view at all times.
Resolution, Resolving Power, and Diffraction Images
These terms form a basic part of the jargon associated with optics and telescopes, a jargon that even the most novice telescope user can understand. Resolution is a qualitative expression of how much detail can be observed through a given telescope.
Telescopes are said to be of high-resolution if they are manufactured to optical standards that permit a level of visible detail consistent with the aperture and optical design of the instrument.
Stars (as opposed to the Moon, planets, or terrestrial objects, for example) are among the most difficult of objects for a telescope to image and bring to a sharp focus, because stars are point-sources of light: from the astronomer’s point of view stars consist of light energy packaged in an infinitesimal area, or point. Surprisingly perhaps, the telescope forms images of stellar point sources as finite-sized discs having real diameters. In other words although nature sends a point-size beam of light to the telescope, the observer looking through the telescope sees not a point-size image, but a tiny disc, called the Airy disc, with faint rings of light surrounding it. This telescopic image of a star, consisting of the Airy disc and its surrounding rings of light, is called the diffraction image.
The concept of the diffraction image is important because it allows the telescope user to rate the quality of the telescope’s optical system. One such rating is determined by the telescope’s ability to clearly separate, or resolve, two star points (i.e., two Airy discs) located very close to each other. The larger a telescope’s aperture, the greater its ability to show two adjacent stars as separate, distinct images, rather than as one overlapping image. This ability is called resolving power. If a telescope’s optical quality permits it to resolve star points to the theoretical limit of its aperture capabilities, then the telescope is said to be diffraction-limited.
The telescope to suit your interests
If you are making the transition from a pair of binoculars to a telescope and have spent some time using the local astronomical society’s instruments you will by now have an inkling of the type of observation that appeals to you — this is not to say that you should be pigeonholed as being a lunar observer or a deep sky observer, rather that you will tend to be drawn in a particular direction.
If, say, you have leanings toward the Moon and bright planets (meaning Venus, Mars, Jupiter and Saturn) then a 9cm (3.5") refractor or a good 11.5cm (4.5") Newtonian reflector will suffice, though a 10cm refractor or 15cm Newtonian would be a better proposition if you are serious about studying them in a some detail. Since you will be dealing with higher magnifications then a good mount is essential: if you have the choice between a weighty and rigid alt-az mount with good slow motion controls and a small equatorial on a light tripod, opt for the former.
Alternatively, you may find that you are attracted to the deep sky — way beyond the fringes of the Solar System into the realm of the stars, nebulae and distant galaxies. If this is the case then the best course of action is to invest in a Newtonian reflector of at least 15cm (6") aperture. For purely visual deep sky use there is little that will match for convenience a short focal length 20cm Dobsonian — it’s capable of showing you most objects of interest, it’s light, manoeuvrable, takes virtually no time to set up and fits neatly on the back seat of the car so you can easily take it out into the countryside for truly dark skies — and you can still use it for the Moon and planets!
Choosing eyepieces
Whichever type and size of telescope that you opt for there are a few essentials that you will need, the first of which are eyepieces. Although you can just about get away with two, a much better proposition would be three: a low power, medium and high power. By low magnification I mean about 30x which will give you fields of view in excess of 1 degree (or two Full Moons side by side) which is ideal for looking at comets, star clusters and nebulae. A medium power eyepiece is about 90x which will show you the Moon nicely cradled in the field of view, while a high power ocular will be around the 150x to 200x mark, capable of showing roughly 2/5ths of the Moon’s disc at once.
This range of magnifications — 30x to 200x — is applicable to our ‘average’ 9cm refractor or 11.5cm Newtonian. Larger instruments can use correspondingly higher powers, but there are practical limits imposed by the steadiness of the atmosphere that set the maximum magnification at 300x or so. At this juncture it is instructive to say that magnification is NOT the be all and end all of owning a telescope — ALWAYS use the minimum magnification that shows you crisply defined detail. If — as is commonly the case — the air is very unsteady making the image of the Moon ripple like looking through running water, then no amount of additional magnification is going to improve the view. Choose a lower power and wait for another night of good seeing.
Types of eyepiece
There are two main things to know about your prospective eyepieces: their design and focal length. The designs to look for are Plossls or Orthoscopics, though the slightly cheaper Kellners are a good standby — particularly if you have a typical Newtonian telescope. My personal preference is for the Plossl design, especially when anti-reflection coated. They possess well-corrected, wide fields of view and are well suited to all types of telescopes and observational subject matter. At roughly £25-£100 each they are not cheap, but most good instruments are supplied with at least one these days. However, when you consider that an eyepiece is just as important a link in the optical chain as the objective mirror or lens, then the cost is put into perspective.
Calculating your eyepiece’s magnification
Upon close inspection of the eyepiece you will see a number engraved or printed on the barrel close to the name — this is its focal length and is of great importance when calculating the power it will deliver with your telescope. Quite simply, the magnification may be found by dividing the focal length of the telescope by this number. For example, a 9cm refractor with a focal length of 1000mm (100cm) will give 40x when used with an eyepiece of 25mm focal length. It follows that eyepieces of shorter focal length give higher magnifications.
It is also worth mentioning here that most modern eyepieces are supplied with a push-fit chromed barrel 31.75mm (1.25") in diameter and are threaded internally to accept coloured glass filters to enhance the view of certain objects. It is wise to buy a telescope with a focuser of such a size, though adapters can be obtained for second-hand instruments. Older telescopes of smaller aperture tended to have 24.5mm (0.965") push-fit barrels, while for larger instruments you will see some 50.8mm (2.0") push-fits for spectacular low power, wide angle views and astrophotographic use.
Astrophotography
For the prospective telescope buyer it is fair to say that for all but the most elementary photography of the Moon an equatorial mount is a necessity. With focal lengths of a metre or more it requires a very well engineered mount to track the stars accurately enough for the images to remain point-like over exposures that may last for ten minutes or more. High resolution lunar and planetary photography calls for critical focusing and rigidity, too.
This is not intended to dissuade the beginner, rather to state that for smaller telescopes the best option is to arrange for a 35mm camera with telephoto or standard lens to be carried ‘piggyback’ fashion on the mount of the instrument which is used visually to monitor the accuracy of tracking. Various prime focus or afocal eyepiece projection adapters can also be purchased from many telescope suppliers that permit the same 35mm camera (minus lens) to be placed at the focus of the main telescope for full or partial disc images of the Moon.
Also, modern digital cameras can be used for astrophotography, combined with the various adaptors available, to achieve astounding results on imaging the Moon and planets. Digital SLR cameras are now widely used for longer exposure photography. For the more serious astro imager, the genuine CCD cameras are now available from around £300.
Finally,
“Talk to Astronomers — not Salesmen”
We at Pulsar are experienced amateur astronomers, and will provide you with sound, professional advice. We also provide you with full support — just a phone call away, with help on all subjects, including setting up, observing and imaging tips and other astronomy related problems. So, browse our website at your leisure, or call us on (0845) 634 9192 if you require further information.